1. Field of the Invention
The present invention relates to a semiconductor device including an oxide semiconductor film and a method for manufacturing the semiconductor device.
Note that the semiconductor device in this specification refers to all devices that can function by utilizing semiconductor characteristics, and electro-optic devices, semiconductor circuits, and electronic appliances are all semiconductor devices.
2. Description of the Related Art
In recent years, a market for mobile communications typified by mobile phones and the like has grown rapidly. To meet increasing requirements such as low power consumption, high integration degree, many functions, and high speed in semiconductor integration circuits that are mounted components, improvements in transistor characteristics are needed.
As one of indexes denoting transistor characteristics, an S-value is given. An S-value represents the amount of a change in gate voltage which is needed for changing a drain current by one digit under constant drain voltage. As the S-value becomes smaller, the controllability of the drain current is increased. Under the present circumstances, it is difficult to make the S-value less than or equal to 80 mV/dec because of miniaturization of transistors and the like.
Attention has also been drawn to a technique by which a transistor is manufactured using an amorphous oxide semiconductor material instead of a silicon-based semiconductor material and is applied to an electronic device or the like. For example, a technique for manufacturing a transistor whose channel layer is formed using an amorphous oxide semiconductor material containing indium (In), gallium (Ga), and zinc (Zn) and having an electron carrier concentration lower than 1018/cm3 is disclosed (see Patent Document 1).